Parkinson's disease is a progressive neurological disease that dramatically alters the ability of individuals to move. The long-term objective of the proposed research program continues to be to understand how Parkinson's disease changes the way muscles are adapted to perform movements. Aim 1 will test the hypothesis that Parkinson's disease causes an inability to: 1) turn off muscles that are activated, 2) prolong muscle activation and delay antagonist muscle activation for longer movements and, 3) use appropriate patterns of muscle activation to adapt to unexpected changes in load. Aim 2 will determine the extent to which the imbalance between flexor and extensor muscles impairs motor function. The hypothesis is that the electromyographic (EMG) abnormalities address in Aim 1 will be greater in extensor muscles than in flexor muscles. Aim 3 will determine whether disease severity influences the patterns of muscle activation observed in patients with Parkinson's disease. The hypothesis is that disease severity does influence patterns of muscle activation. A second hypothesis is that certain EMG deficits will appear successively in all subjects, while others will appear in a different order in different subjects related to the particular manifestation of the disease. For example, the Principal Investigator expects to see a loss of agonist EMG burst during modulation followed by a decrease in latency of the first antagonist EMG burst. In contrast, the difficulty in turning off muscle activation may not emerge in a fixed relation to the loss of agonist during modulation. Studying two groups of patients who have different degrees of disease severity will test both these hypotheses. The experiments have been carefully chosen to build upon prior studies of muscle activation patterns in neurologically normal individuals, and in patients with Parkinson's disease. Aims 1 and 2 will provide information that can be used to develop models of motor control that apply to a wide variety of movements in Parkinson's disease. Aim 3 will allow the investigators to determine the extent to which EMG and performance deficits manifest themselves at all stages of the disease. Understanding how much muscle activation patterns change to perform movements is important in evaluating pharmacological, neurological, as well as physical therapy interventions that are designed to facilitate movement in Parkinson's disease.